Rock bolts are utilised to secure the roof or walls of an underground mine, tunnel or other ground excavation by inserting the rock bolt into a bore hole drilled in the face of the rock to be secured and securing the rock bolt within the hole. One known form of rock bolt is a cable bolt (sometimes called a strand bolt) that includes a plurality of helically wires wound into the form of a cable.
In one known cable bolt installation method, grout is injected into the bore hole after insertion of the cable bolt so as to encapsulate at least part of the length of the cable, securing the cable bolt within the bore hole once the grout has set. A rock face plate is attached to the exposed end of the cable bolt and arranged to bear against the rock face to thereby apply a compressive load against the rock face, stabilizing the same. In order to increase the bond strength between the cable bolt and the grout encapsulating the cable bolt, it is known to form one or more bulbous portions in the cable bolt, thereby locally increasing the cross sectional area of the cable bolt, acting to resist pulling of the cable bolt through the grout.
In one previously proposed technique of forming bulbous portions along the length of a cable bolt, as described in Australian Patent No. 640,906, a pair of longitudinally spaced clamps are arranged to releasably clamp a cable, with one of the clamps being longitudinally displaceable in relation to the fixed clamp by way of an hydraulic ram. In operation, the cable is clamped with the two clamps in a spaced apart relationship, with the displaceable clamp then being displaced toward the fixed clamp, thereby buckling the wires of the cable between the clamps, resulting in the formation of a bulbous portion in the cable bolt.
Given the strength of the wires forming the cable bolt, and the fact that they are supported against buckling by virtue of the tightly wound arrangement of the wires of the cable bolt, large compressive loads are required to initiate the buckling. As a result, large clamping loads are required to prevent slippage of the clamp jaws on the cable. This necessitates powerful rams to drive the clamps.
The displaceable clamp is also utilized to advance the cable bolt after having formed a bulbous portion in preparation for forming a further bulbous portion further along the cable. The cable is advanced through a sequence of unclamping the clamps after forming the first bulbous portion, retracting the displaceable clamp, reclamping the cable bolt with the displaceable clamp and subsequently advancing the displaceable clamp toward the fixed clamp. As a result of this post-bulbous portion formation sequence being required to advance the cable, production is relatively slow.